This invention relates to a system for controlling a safety device for a vehicle such as air bag device, pretensioner of a seat belt or the like.
Recently, in order to ensure the safety of passengers, as well as a driver, of a vehicle, various safety devices such as an air bag device, a pretensioner of a seat belt and the like are developed. It is demanded for any of these safety devices that they are assuredly activated at a good timing when a vehicle collision has occurred, and that they are not activated accidentally when a vehicle is running in a normal condition.
In a conventional system for controlling an air bag device, a squib drive circuit is provided between a battery which is mounted on a vehicle and the ground. This squib drive circuit comprises a transistor (switching means) and a squib (activator) for an air bag, both arranged in series. When electric current is supplied to the squib, the latter is ignited to inflate or expand the air bag.
The above control system further comprises an acceleration sensor for detecting an acceleration of the vehicle, an integral means for integrating a signal from the acceleration sensor, and a collision judgment means for making a judgment that a vehicle collision has occurred and outputting a trigger signal when the integral value exceeds a trigger threshold level. The transistor is turned on in response to the trigger signal outputted when the vehicle collision has occurred and supplies electric current to the squib. It is necessary that the trigger threshold level is set to a proper level. The reason is that if the trigger threshold level is too low, there is a possibility that the air bag will be accidentally expanded during a normal running condition of the vehicle, and on the other hand, if the trigger threshold level is too high, there is a possibility that the air bag will not be expanded when a vehicle collision has occurred.
In the cases where a center pole collision (only the center of a front portion of the vehicle has collided with a pole) and an under-ride collision (a kind of collision in which the vehicle climbs over an obstacle) have occurred (in these cases, the air bag is required to be expanded), the degree of deceleration is small at the time immediately after the vehicle collision has occurred and the degree of deceleration is abruptly increased from a certain time point. Therefore, there are problems which are difficult for the conventional control system to overcome. Specifically, in a vehicle collision of the type mentioned above, the degree of deceleration of the vehicle speed is small immediately after the vehicle collision has occurred and therefore, an integral value of a corresponding acceleration signal is also small. This means that it takes a longer time for the integral value to exceed the trigger threshold level than other types of vehicle collision. The result is that the supply of electric current to the squib is delayed which in turn delays the expansion of the air bag from a desired good timing.
Japanese Laid-Open Patent Application No. Hei 3-243445 (corresponding to U.S. Pat. No. 5,262,949) discloses a technique for adjusting a trigger threshold level in accordance with the acceleration and speed of a vehicle. However, no consideration is paid to the frequency of the acceleration signal as in the present invention.